The hadith I am referring to is:
If a young man repents, the torment of grave is withheld from east to west for 40 days.
What is the authenticity of this hadith?
According to islamqa, the prophet ( PBUH ) didn't say this and it is not mentioned in any books of hadeeth. It is also not known that any of the scholars narrated it, even if it is a weak hadeeth.
In Hindi: Nabee Muhammad ne yah nahin kaha aur yah hadees kee kisee bhee kitaab mein ullikhit nahin hai. yah bhee gyaat nahin hai ki kisee bhee vidvaan ne ise sunaee hai, bhale hee vah ek kamajor hadees hai.
A question was asked on IslamWeb, regarding the authenticity of this so called Hadith, they responded that they haven't heard of it and couldn't find it in any of their collections:
Q: What is the authenticty of these ahadith: 1. If a young man repents, torment of grave is withheld from east to west for 40 days.
We have not found the two Ahaadeeth which you mentioned in the question in the books of Sunnah that we have at hand despite the large number of those. We have not even found them in the literature of Dha’eef (weak) and fabricated Ahaadeeth.
One should be careful not to attribute a saying to the Prophet sallallaahu alayhi wa sallam ( may Allaah exalt his mention ) which he did not say. While commenting on the Saheeh (sound) Hadeeth which reads: "Whoever says a Hadeeth on my behalf while he knows that it is a lie, then he is one of the two liars (i.e. either the one who invents a lie or the one who repeats it,; both are liars).” Imaam Ad-Daarimi said: “If a man narrates a Hadeeth while he does not know any evidence that it was said by the Prophet sallallaahu `alayhi wa sallam ( may Allaah exalt his mention ) and he relates such a Hadeeth, then I fear that he is included in this Hadeeth (i.e. that he is one of the two liars).” [End of quote]